The Nervous System. 261 



those parts where the greatest amount of movement occurs, 

 as in the neck. The cord is not the same shape nor the 

 same size throughout ; oval in the cervical region, it be- 

 comes circular in the dorsal, and again oval in the lumbar 

 portion. It is largest where any considerable bulk of 

 nerves is being given off, and thus there is an enlargement 

 corresponding to the fore, and another to the hind limbs. 

 We speak of the superior and inferior face of the cord, or 

 perhaps it would be more in accordance with comparative 

 anatomy to describe it as the dorsal and ventral face or 

 surface. On exposing the spinal canal, a large number of 

 nerves are found to be passing through the dura mater, 

 either outwards or inwards, and these gain an exit from or 

 entrance to the spinal canal by means of the foramen formed 

 at the juncture of the vertebras. 



On opening the dura mater, it is easy to determine that 

 the nerves divide in such a way that part of them run to 

 the upper or superior surface of the cord, and part to the 

 inferior. These are spoken of as the superior and inferior 

 spinal nerves. In the horse the number of branches of 

 nerve thus formed is considerable, for both the superior 

 and inferior enter the cord not by a single root, but by 

 several. On the superior roots, but outside the dura mater, 

 is found a nervous body termed a ganglion ; each of the 

 branches of the several roots of a superior spinal nerve has 

 a ganglion on it. No such ganglion exists on the inferior 

 root, but inferior and superior roots unite after passing 

 the ganglion to form a mixed spinal nerve (see Fig. 24). 

 The function of these two sets of nerve roots is entirely 

 different. The superior root, containing the ganglion, 

 conveys sensation and sensation only ; the inferior roots 

 convey motion, in addition to certain other functions to be 

 shortly alluded to ; the superior roots are passing into the 

 cord, the inferior roots are passing out of it. Passing out 

 with the inferior root of the spinal nerve, but indistinguish- 

 able from it, is a branch of nerve known as the white ramus 

 communicans, which leaves the main trunk after the mixed 

 nerves have formed, and runs to a distinct system known 



